Climate Proxies
How can you measure the climate
of the past?
Learner outcomes
At the end of this lecture you should be able to
1. Describe how proxies differ from observations
2. Describe how tree rings, corals, fossils, lake ice
and lake duration are used to estimate local
climate
3. Describe how ocean sediment and ice cores are
used to estimate global climate
4. The difference between stable and radiometric
isotopes and what type of information they tell use
about past climates
Proxies
Unlike instrumental records that tell us only
about the most recent century, proxy
records (natural archives of climate
change) enable us to place recent climatic
change in the context of the last several
hundred to thousand years.
1000 Years of CO2 and
Global Warming
Temperature
(Northern Hemisphere)
CO2 Concentrations
How do we develop proxies?
• Assumptions
• Observations of phenomenon today
• Link current observations to past records
Assumptions
• Observation: Solar radiation varies but
overall decreases
• Therefore solar radiation in the past was
higher
Proxies
• Corals*
• Tree rings*
• Pollen*
• Fossils*
• Sea level
• Lake ice duration*
• Ocean sediments
• Ice Cores
*indicative more of local climate change than global
climate change
Corals
• Shells made of Calcium Carbonate
(CaCO3)
• Shell in equilibrium with the ocean water
• Band width provided evidence of
temperature the coral grew in
• Growth rates change with ocean
temperatures, pH levels
• Local climate
Why should we hug trees?
• Dendrochronology is the study of the annual variability
of tree ring widths, which can be extended back to
8000 years ago.
• The study of trees provides climate information
regarding temperature, runoff, precipitation, and soil
moisture.
• Local climate
Date of last ring is
year tree was cut
1930
1890
1870
1910
1950 1970
Tree Rings
Growth conditions recorded in rings
• Wide ring-warm days sufficient water
• Narrow-cold days/drought
What can plant and animal fossils tell us
about ancient climates?
These 350 Ma fossil ferns were most likely the
oldest on land, and likely required high pCO2
levels.
A trilobite, the
three- lobed king
of warm, shallow
Cambrian seas
• Certain plants and
animals live only in
specific environments,
so their presence is a
clue to local climate.
Soft-bodied Waptia, an
arthropod from the
Cambrian Burgess
Shale
Lake Monona Ice Duration 1855-2005
180
Duration of Ice (days)
160
140
120
100
80
60
40
20
0
1855
1875
1895
1915
1935
Seasons
Source: Wisconsin State Climatology Office
1955
1975
1995
Examples of Climate Proxies
• Pollen
• Tree Rings
• Ice Cores
Lake Ice Duration
Lake Ice Thickness
Global Proxies
• Sea level
• Ocean sediment
• Ice cores
– Layers (varves) in ice cores
– Gases in ice cores
– Stable Isotopes: O-16 to O-18 ratio in ice
cores
– Radiometric Isotopes:Carbon dating of
sediment in the ice cores or glacial deposits
Sea Level
• Glaciation –low sea level
Ocean Sediment Cores: 3-3.5km
• Thick levels of
sedimentation can
indicate heavy
weathering, warmer
temperatures
• Volcanic sediments
• Loss of sediment
layers through
erosion
• 55 mya
Frozen Core
Some cores go 3 km deep!
•
Vostok, Antarctica
78°28' S, 106°48'E: Coldest
Places on Earth
Vostok Station
Nationality: Russia
Location: Vostok - an outpost if there
ever was one - is located near the
The coldest
South Geomagnetic Pole, at the center
recorded
of the East Antarctic ice sheet, where
temperature on
the flux in the earth's electromagnetic
Earth, -128.6°F (field is manifested.
89.2°C) was
measured here on
July 21, 1983.
Ice core drilling 3.4
km to go ½ million
years into past
climate
Ice Cores: Varves
• A varve is an annual layer of sediment or
sedimentary rock
Section of Greenland Cores
Dozen Ice Ages going back 1 billion years
How can ancient greenhouse gases be
trapped?
• Atmospheric gases (CO2, CH4, SO2, etc.) can be trapped
in glaciers as frozen water metamorphoses from snow
to firn to recrystallized ice.
During the Last Glacial
Maximum pCO2 is
estimated at 180 ppm
The record of atmosphere
CO2 since the Industrial
Revolution
Ice Core Thermometer
Isotopes
• Stable Isotopes-temperature
• Radiometric dating-rate and date
How can oxygen isotopes used as
paleoclimate proxies?
• isotope -- atoms of the same element with the same
atomic number (chemical properties) but differing
atomic weight (physical properties). Differ in number of
neutrons.
• Oxygen is composed mostly of 16O and 18O, which as
part of water molecules are separated by physical
processes.
A typical carbon
atom with 6 protons
and 6 neutrons and
6 electrons.
Fractionation
• To divide or separate into parts
• Ocean water is made up of both O-16 and
O-18
There is a standard or average ratio of O-18
to O-16 (standard mean ocean water as
the baseline, SMOW)
• Certain physical and biological processes
change the ratio (this is fractionation)
Oxygen Isotopic Ratios or
Amounts
O18/O16 ratio in glacial ice indicate the
atmosphere temperature in which the
snow that made up the ice formed
Extent of isotopic difference (fractionation) is
dependent on the temperature.
So they form a temperature proxy!
Oxygen Isotopic Ratios vs.
Amounts
O18/O16 ratio versus O-18 and O-16
Usually described as a ratio
Oxygen Fractionation Summary
• If ratio O-18 to O-16 is higher than
expected in the ocean, colder
temperatures
• If ratio O-18 to O-16 is lower than
expected in the ocean, warmer
temperatures
Oxygen Isotopes in Glacier Ice
• Polar ice is preferentially enriched with O16 relative to the ocean (O-16 locked in
glacier ice). So especially during glaciation
ocean water is “heavy”
• Why is glacier ice “light”?
– The water source is from precipitation which
is preferentially light.
• So during a glaciation you would expect
remaining ocean water to be heavy
Radiometric Isotopes
• Isotopes that decay (Carbon) can tell us
the approximate date of an event or the
rate at which an event took place
– Glacial retreat
Carbon Dating: Rate of Glacial Retreat
Greenland Ice Sheet and Arctic:
Northern Hemisphere
Antarctica: South Pole